Currently, driving circuits of lasers are mostly driven by DC circuits directly. Such a driving circuit usually includes a transistor or MOSFET, a power supply, a laser, and a current limiting resistor connected in series. A transistor D1, a MOSFET, and a current limiting resistor R1 are connected in series and thereby forming the complete branch circuit. Thus, if a voltage Vc is applied to the branch circuit, the total voltage Vc is equal to the sum of the voltage VLD distributed to the transistor D1, the voltage Vmos distributed to the MOSFET, and the voltage VR1 at the current limiting resistor R1. The power supply voltage is distributed over the whole branch circuit. The power supply voltage is mainly 3.3 V or 5 V. The forward driving voltage of the laser is not greater than 2.5 V. As a result, at least 0.8 V or 2.5 V of voltage is distributed to the transistor or the MOSFET 11 and the current limiting resistor R1, thereby causing 24.24% or 50% of the power consumption is wasted respectively.